JPH069262B2 - Superconducting device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting device that operates at cryogenic temperatures, and more particularly, to control the number of superconducting and normal-conducting elements tunneling in a semiconductor by the voltage applied to a control electrode. Superconducting switching device.

[Background of the Invention]

As a superconducting device using a semiconductor and having an electrode structure for controlling the characteristics of the device, T.W. D. Clark
Proposed by JOFET (J. Appl. Phys.
2736, 51 (1980)) is known. The JOFET has a problem that the current gain cannot exceed 1 because the Cooper pair is supplied from the gate electrode.
Therefore, the circuit gain is small. As a device of the same kind, a superconducting device described in JP-A-57-176781 is disclosed. In this device, the superconducting transition temperature Tc
A semiconductor material that can become superconducting at the following temperature is used, but Tc in this case is generally the liquid He temperature (4.2 ° K).
However, it was necessary to cool it to 4.2 ° K or less in order to operate the circuit stably.

As described above, none of the conventional superconducting devices having a control electrode is satisfactory in terms of circuit gain and circuit stable operation.

[Object of the Invention]

An object of the present invention is to provide a high-speed switching element whose characteristics can be controlled by voltage and which has a large current gain.

[Outline of Invention]

In order to achieve this object, the present invention provides a first and a second superconducting electrodes provided on a semiconductor made of a normal conducting material which always maintains a normal conducting state at a use temperature, and a first and a second superconducting electrodes. A control electrode of normal conductivity or superconductivity formed on the upper or lower part of the semiconductor between the two via a dielectric film, and a voltage applied between one of the first or second superconducting electrodes and the control electrode. It operates by changing the superconducting weak coupling state between the first and second superconducting electrodes. That is, the distance l between two superconducting electrodes formed on a semiconductor (layer or substrate)
To a degree (0 <l ≦ 300 nm) in which a Josephson current flows due to a superconducting tunnel between the electrodes, and the semiconductor is used as a tunnel barrier layer for the superconducting tunnel, and an insulating film having a thickness of about 10 to 30 nm is formed on the semiconductor. It is characterized in that the device is operated by changing the width and height of the tunnel barrier by changing the amount of space charge in the semiconductor by the voltage applied to the control electrode provided via. Therefore, the semiconductor need not be used below its superconducting transition temperature. Further, since the control electrode is not used for supplying the Cooper pair, the device gain can be increased.

Example of Invention

 Hereinafter, the present invention will be described in detail with reference to examples.

FIG. 1 shows a first embodiment of the present invention. Impurity concentration 10 ¹⁵
cm ^-3 impurity B in the substrate 1 made of the following n-type Si 10 ¹⁸
The p-type control electrode 2 having a depth of 1 to 2 μm was formed by introducing ˜10 ²⁰ cm ⁻³ . It is desirable that the substrate 1 is made of a material that is semi-insulating at such an extremely low temperature. Then
The surface of the substrate 1 was oxidized to form an insulating film 3 of SiO ₂ having a thickness of about 30 nm. Then, a semiconductor layer 4 made of Si and having a thickness of about 100 nm was formed by a vapor phase growth method or a molecular beam growth method. The semiconductor layer 4 was able to achieve the object of the present invention in both the amorphous state and the polycrystalline state. An n-type impurity (As or P) was introduced into the semiconductor layer 4 at a concentration of 5 × 10 ¹⁹ to 10 ²⁰ cm ⁻³ . Subsequently, strip-shaped first and second superconducting electrodes 5 and 6 having a line width of about 5 μm are formed on the same side of the substrate 1. It is desirable that the material constituting the superconducting electrode is selected from Pb, an alloy containing Pb as a main component, Nb, and an Nb compound so that the element can be operated in liquid helium, but the material is not limited to this. There is no. Through the above steps, the superconducting device of the present invention could be manufactured.

In this device, the superconducting electrodes 5 and 6 are connected by a superconducting weak bond when cooled below the transition temperature of the superconducting electrode material, and therefore the maximum Josephson current Im flowing between the two superconducting electrodes. Is given by Im = 4πΔ / 2eR _N. Here, Δ is the gap energy of the superconducting electrodes 5 and 6, e is the elementary charge, and R _N is the normal conducting tunnel resistance of the superconducting weak bond. The distance between the two superconducting electrodes 5 and 6 is 300 nm to form a superconducting weak bond.
It is desirable that both electrodes are spatially separated as selected below.

When a negative voltage is applied to the control electrode 2 with respect to the superconducting electrode 5 or 6, a positive charge is induced on the semiconductor layer 4 side at the interface between the semiconductor layer 4 and the insulating film 3, and this charge Therefore, the state as a tunnel barrier changes, and R _N changes to a larger value, so that the maximum Josephson current Im that can flow without generating a voltage between the electrodes 5 and 6 decreases.

FIG. 2 is a diagram for explaining the characteristics of such a superconducting device according to the present invention. When a load is provided as in this figure, the operating point at point A when the voltage Vc of the control electrode 2 is 0 is the gate voltage. The signal is switched to point B by the application of a signal of V _G <0. In this case, since the control electrode 2 is separated from the semiconductor layer 4 by the insulating film 3, the device operates in the voltage control type. In the embodiment shown in FIG. 1, the control electrode 2 made of p-type semiconductor and the n-type semiconductor layer 4 are used. Instead of these, the control electrode 2 made of n-type semiconductor and the p-type semiconductor layer 4 are used. Is also good. In addition to Si, G is used as the material of the semiconductor layer 4.
You may use e, GaAs, InAs, InP, InSb, etc. The material of the insulating film 3 is SiO or S.
The same effect could be obtained by using a thin film of i ₃ N ₄ . In this case, Ge has a carrier concentration of 6 × 10 ¹⁸
cm ^-3 or more, the carrier concentration is 1 × 10 ¹⁷ cm ^-3 or more for GaAs and InP, and 1 × for InAs and InSb.
It is preferable that it is 10 ¹⁶ cm ⁻³ or more for the device to operate at an extremely low temperature. However, even if the carrier concentration is less than the numerical value shown here, if the voltage applied to the control electrode is increased, I was able to reach my goal.

In this embodiment, the substrate 1 has an n-type semiconductor and the semiconductor layer 4
Although an n-type semiconductor material was used for the
Similar effects could be obtained when a positive voltage was applied to the control electrode 2 using a p-type semiconductor for the semiconductor layer 4.

Band diagrams between the first and second superconducting electrodes of the superconducting device shown in FIG. 1 are shown in FIGS. 5, 6, 7, and 8.

In each figure, 7 is the conduction band of the first superconducting electrode, 8 is the forbidden band of the first superconducting electrode, 9 is the filling band of the first superconducting electrode, 10 is the conduction band of the second superconducting electrode, and 11 is the conduction band. The forbidden band of the second superconducting electrode, 12 is the filling band of the second superconducting electrode.

FIG. 5 shows the case where a non-degenerate material is used for the semiconductor, and FIG. 6 shows the case where the degenerate semiconductor is used for the semiconductor, and no voltage is applied to the third superconducting electrode (control electrode). Third
The band diagram when the electrode voltage is applied is as shown in Fig. 7 when the semiconductor is non-degenerate, and as in Fig. 8 when it is a degenerate semiconductor. In the former, the superconducting critical current increases and in the latter, the superconducting critical current. Is reduced and switching is performed.

FIG. 3 shows a second embodiment of the present invention. The control electrode 2 was formed by depositing Nb, which is a superconducting metal, to a thickness of about 200 nm by a sputtering method using Ar gas, and processed by a reactive ion etching method using CF ₄ gas. Amorphous Si with a film thickness of about 50 nm was then formed on the surface by atmospheric pressure CVD.
An O ₂ film was deposited to form an insulating film 3. Subsequently, an amorphous silicon film having a thickness of about 200 nm containing phosphorus at a high concentration of 10 ¹⁹ cm ⁻³ or more was formed by RF discharge of silane gas to form a semiconductor layer 4. Finally, the superconducting electrodes 5 and 6 made of Nb having a thickness of about 300 nm were formed by the sputtering method and processed by the reactive sputtering method. In this embodiment, the structure itself of the superconducting device is the same as that of the first embodiment, but a superconducting metal is used for the control electrode 2. Although it is desirable to use a self-oxidizing film of the superconducting metal for the insulating film 3, SiO ₂ ,
A thin film of SiO, Si ₃ N ₄ or the like may be deposited and used. As the superconducting metal, a material selected from at least one of Nb, a compound of Nb, and Ta is used.
It is desirable to increase the relative permittivity of, but not limited to this. As a result, the operating voltage of this device can be lowered and the power consumption can be reduced.

FIG. 4 shows a third embodiment of the present invention. In this embodiment, the semiconductor layer 4 is made of Si single crystal. This Si single crystal is p-type or n-type and has an impurity concentration of 1 × 10 ¹⁹ cm ^-3.
The following is desirable.

That is, a Si single crystal having a (100) orientation is processed by anisotropic etching with KOH using SiO ₂ or the like as a mask as shown in FIG. 4 to form an extremely thin semiconductor layer 4 having a thickness of about 100 to 200 nm. did. The surface of the semiconductor layer 4 is oxidized to form an insulating film 3, and Pb which is a superconducting metal is deposited as the control electrode 2 by a vapor deposition method to a thickness of about 500 nm.
The processed one was used. Next, the upper surface of the semiconductor layer 4 was cleaned and the superconducting electrodes 5 and 6 made of Nb or Pb having a thickness of about 300 nm were formed and processed by the reactive ion etching method. Even with such a structure, the superconducting device of the present invention could be realized.

In the first and second embodiments, the control electrode 2 is provided below the semiconductor layer 4 (which may be a semiconductor substrate) between the first and second superconducting electrodes 5 and 6 with the insulating film 3 interposed therebetween. However, the same effect can be obtained even if the control electrode 2 is provided above the semiconductor layer 4 between the first and second superconducting conductive electrodes 5 and 6 via the insulating film 3.

The superconducting device manufactured as described above has a very small hysteresis of characteristics because the electrostatic capacity existing in parallel with the superconducting weak coupling is small, which speeds up the conventional Josephson circuit and simplifies the circuit. It is possible to use a direct current power source instead of the alternating current power source which is an obstacle to the conversion. Further, even a device that is a voltage control type and uses a semiconductor, since the tunnel effect is utilized, the high frequency response is not limited by the mobility of the semiconductor carrier, and high speed switching can be realized. did it.

〔The invention's effect〕

As described above, according to the present invention, the characteristics of the Josephson junction element can be changed by the control by the voltage, and this can be used as the switch. Therefore, according to the present invention, it is possible to obtain an ultra-high speed switching circuit having a large circuit gain. Further, it can be mixed with a circuit manufactured using a Josephson junction element.
Therefore, there is an effect that the conventional computer system using the Josephson junction element can be configured more easily.

[Brief description of drawings]

1, 3, and 4 are cross-sectional views each showing a part of the superconducting device according to the embodiment of the present invention, and FIG. 2 is a diagram for explaining the characteristics of the superconducting device according to the embodiment of the present invention. FIG. 8 to FIG. 8 are views each showing an electronic band state of the superconducting device according to the embodiment of the present invention. 1 ... Substrate 2 ... Control electrode 3 ... Insulating film 4 ... Semiconductor layer 5, 6 ... Superconducting electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ushio Kawabe 1-280, Higashi Koigakubo, Kokubunji, Tokyo Metropolitan Research Laboratory, Hitachi, Ltd. (56) Reference JP-A-57-176781 (JP, A) JP-A-59 -103389 (JP, A)

Claims (16)

[Claims] 1. A semiconductor layer made of a normal conducting material which always maintains a normal conducting state at an operating temperature, and first and second superconducting materials provided on the semiconductor layer with a predetermined distance therebetween.
A superconducting electrode and a control electrode made of a normal conducting material or a superconducting material provided on the semiconductor layer between the first and second superconducting electrodes via an insulating film. 2. The control electrode is formed on at least the semiconductor layer between the first and second superconducting electrodes with the insulating film interposed therebetween. A superconducting device according to the item. 3. The control electrode is formed at least under the semiconductor layer between the first and second superconducting electrodes via the insulating film. A superconducting device according to the item. 4. The superconducting device according to claim 1, wherein the semiconductor layer is made of at least one material selected from a single crystal semiconductor, a polycrystalline semiconductor and an amorphous semiconductor. 5. The self-oxidizing film of Si formed on the surface of the semiconductor layer or the surface of the control electrode, or the SiO ₂ film deposited on the surface of the semiconductor layer or the surface of the control electrode, SiO The superconducting device according to claim 1, wherein the superconducting device is a film or a Si ₃ N ₄ film. 6. The control electrode is Nb, a compound of Nb, Ta
Of at least one of the above materials, and the insulating film is a self-oxidized film of the above material, or a SiO ₂ film, or a SiO ₂ film.
The superconducting device according to claim 1, wherein the superconducting device is formed of a film or a Si ₃ N ₄ film. 7. The superconducting device according to claim 1, wherein the semiconductor layer is made of Si single crystal. 8. The superconducting device according to claim 1, wherein the first and second superconducting electrodes are provided on the semiconductor layer at an interval of 300 nm or less. 9. The superconducting device according to claim 1, wherein the insulating film has a thickness of 10 nm to 50 nm. 10. A substrate is provided under the semiconductor layer with the insulating film interposed therebetween, and the control electrode is embedded in the substrate so as to be in contact with the insulating film. The superconducting device according to item 1. 11. The superconducting device according to claim 10, wherein the control electrode is provided by introducing a predetermined impurity into the substrate made of a semiconductor. 12. A substrate is provided under the semiconductor layer with the insulating film interposed therebetween, and the control electrode is formed on the substrate so as to be in contact with the insulating film. The superconducting device according to item 1. 13. The superconducting device according to claim 1, wherein the semiconductor layer is a thin-film semiconductor layer provided on a substrate. 14. The superconducting device according to claim 1, wherein the substrate is electrically insulated from the semiconductor layer. 15. The superconducting device according to claim 14, wherein the substrate is made of an insulating material. 16. The substrate having an impurity concentration of 10 ¹⁵ cm ⁻³ or less n
15. A superconducting device according to claim 14, characterized in that it is made of p-type or p-type Si.